Reading and Building a Formulation

What a formulation is

A formulation, or recipe, is the complete list of what goes into a compound and how much of each. It is the single most important document in rubber technology, because everything else — processing behaviour, cured properties, cost, service life — follows from it.

It is also compact. A complete recipe for a demanding automotive part might have twelve to eighteen lines. Learning to read one quickly is a skill worth acquiring early, because it lets you understand a compound faster than any amount of test data will.

The standard order

Recipes are conventionally written in the order ingredients are added during mixing, which is also roughly the order of decreasing quantity. That convention makes them comparable at a glance.

Curatives come last because they are added in a separate, cooler mixing stage. Putting them anywhere else in the list would misrepresent how the compound is actually made.

An annotated tread compound

Here is a passenger tire tread recipe with every line explained.

The oil level is worth a note, because 30 phr looks high against a general guideline. It tracks the 70 phr of black — a heavily reinforced tread needs that much oil to remain processable. In practice much of it often arrives with the polymer, since oil-extended SBR grades are supplied with oil already incorporated, and a recipe may show it either way. Read it as a set of decisions. Two polymers because neither alone gives both grip and wear. A hard black at high loading because tread wear is the dominant requirement. Aromatic oil because the polymers are diene rubbers and colour does not matter. A full antidegradant package because the part lives outdoors. And a semi-EV cure with a sulfenamide because the compound goes through a long, hot process before it reaches the mould.

One currency note: 6PPD remains the standard antiozonant for tyre compounds, but its ozonation product has been linked to acute aquatic toxicity, and tyres containing it are under active regulatory review with alternatives assessments underway in some jurisdictions. The compounding logic here is unaffected; the ingredient choice may not survive the decade.

An annotated seal compound

A very different application produces a very different recipe.

Notice what is absent. There is no antiozonant, because EPDM does not need one — its saturated backbone provides the protection chemically that a diene rubber must buy with additives. And notice the three accelerators totalling 3 phr, which is what low unsaturation costs. The very high filler and oil loading is EPDM's characteristic advantage: it accepts extension that would ruin most polymers, which is why weatherstrip is cheap.

Units and what phr does not tell you

phr is a weight ratio. It says nothing directly about volume, and volume is what determines how many parts you get from a batch.

To convert, you need specific gravity. Every ingredient has one, and the compound's specific gravity is calculated from the weighted contribution of each.

The spread matters. Zinc oxide is six times denser than the polymer, and calcium carbonate is three times. A compound heavily loaded with mineral filler is substantially denser than one filled with black at the same phr, and denser still than an unfilled gum. Two compounds at the same cost per kilogram can differ noticeably in cost per part.

Batch weight

A mixer has a fixed chamber volume, so a batch is sized by volume rather than by weight. The calculation is simple and worth knowing.

Batch weight equals chamber volume multiplied by compound specific gravity multiplied by a fill factor — typically around 0.70 to 0.75, because the mixer needs free space to work.

A consequence that catches people out: changing a formulation changes the batch weight. Add 20 phr of calcium carbonate and the compound gets denser, so the same mixer now takes more kilograms per batch. If the batch card was written in kilograms and never updated, the mixer is now underfilled and dispersion suffers.

This is why batch cards are recalculated when a formulation changes, and why a formulation change that looks trivial on paper can produce a mixing problem nobody anticipated.